Novel Electrochemical Flow Sensor Based on Sensing the Convective

Article Cite This: ACS Sens. XXXX, XXX, XXX−XXX

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Novel Electrochemical Flow Sensor Based on Sensing the Convective-Diffusive Ionic Concentration Layer Sinwook Park, Ramadan Abu-Rjal, Leon Rosentsvit, and Gilad Yossifon* Faculty of Mechanical Engineering, Micro- and Nanofluidics Laboratory, Technion − Israel Institute of Technology, Technion City 3200000, Israel

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ABSTRACT: Presented is a novel flow sensor based on electrochemical sensing of the ionic concentration-polarization (CP) layer developed within a microchannel-ion permselective membrane device. To demonstrate the working principle of the electrochemical flow sensor, the effect of advection on the transient and steady-state ionic concentration-polarization (CP) phenomenon in microchannel-Nafion membrane systems is studied. In particular, we focused on the local impedance, measured using an array of electrode pairs embedded at the bottom of the microchannel, as well as the total current across the permselective medium, as two approaches for estimating the flow. We examined both a stepwise application of CP under steady-state flow and a stepwise application of flow under steady-state CP. KEYWORDS: flow sensor, ionic concentration-polarization, local impedance, perm-selective membrane, microchannel-membrane device

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Current through a permselective membrane under an applied voltage is characterized by the formation of depleted and enriched ionic concentration regions at the opposite ends of the membrane, a phenomenon termed concentration polarization (CP).10−17 Numerous theoretical and experimental studies have addressed the CP phenomenon and the associated current (i.e., Ohmic, limiting, overlimiting) regimes across ion permselective media (i.e., membranes/nanochannels).18−27 The CP phenomenon in microchannelnanochannel/membrane systems has attracted significant attention in relation to various other applications, e.g., solute preconcentration, fuel cell, and desalination.28−30 Net flow across (i.e., normal flow) ion-permselective membranes significantly affects CP behavior and results in shortening (expansion) of the depletion layer length upon flow into (out of) the membrane, as well as a linear to exponential shift in the profile of the one-dimensional steady-state solution.31−33 The effect of normal flow on transient CP behavior was also recently studied.34 In a different setup,35 where the flow direction was tangential to an impermeable membrane instead of normal to it, the CP layer under shear flow behaved as a convective-diffusive boundary layer. In either case of normal or tangential flow, deformation of the depletion layer due to convection is electrically measurable and can be correlated with the flow rate, thus, providing the basis for a CP-based

icroflow sensors using microelectromechanical system (MEMS) technology have been extensively investigated for various flow sensing applications, due to their remarkable advantages such as low energy consumption, a satisfactory degree of sensitivity, a faster response, and easy integration to integrated circuit (IC)-based devices compared to macroflow sensors.1−3 Current commercial MEMS-based solutions for flowmeters capable of measuring small flow rates (μL/min) rely mainly on thermal sensing, which detects temperature changes due to heat advection from a heating element.2−7 However, this approach suffers from signal reduction arising from heat loss through the channel walls to the environment and large axial diffusion due to high thermal diffusivity.8 An alternative method examined herein is electrochemical sensing, which measures ion conductivity changes instead of temperature changes. Unlike the latter, conductivity changes are confined to the fluid and are less smeared due to smaller ionic relative to thermal diffusivity. Previous models of electrochemical flow sensors, using pulses of electrochemically produced tracers (through electrolysis of water), used either downstream oxygen8 or conductivity9 sensors. However, these flow sensors focused only on time-of-flight sensing distal to the electrochemically produced changes. Here, we introduce a novel electrochemical flow sensing method by using both an array of local impedance sensors positioned near an ionic permselective membrane that generates a concentration-polarization layer, as well as current measurements across the membrane. © XXXX American Chemical Society

Received: March 1, 2019 Accepted: June 17, 2019 Published: June 17, 2019 A

DOI: 10.1021/acssensors.9b00431 ACS Sens. XXXX, XXX, XXX−XXX

Article

ACS Sensors

Figure 1. Nafion-microfluidic chip for local impedance measurements. (a) Schematic of the chip consisting of a main microchannel, two side channels, an interconnecting Nafion membrane, and an embedded microelectrode array for EIS measurements. (b) Scaled-up microscope image of the Nafion interface. (c) Schematic of the CP behavior (depletion layer propagation/deformation) within the microchannel, occurring at the microchannel−Nafion membrane interfaces, under various flow rate intensities, with directions from left to right. u0: zero flow condition (u0 ≈ 0), u1−u3: net flow conditions (u1 < u2 < u3).

flow sensor. This work will examine electrical sensing of the ionic CP layer that is generated at the microchannel-ionic permselective membrane interface under tangential flow conditions. Local electrical measurements can be performed by measuring the local solution conductivity within the CP layer. However, such measurements cannot be made under a DC field, due to screening of the electrodes and possible Faradaic reactions. Hence, we instead performed alternatingcurrent (AC) measurements via local electrical impedance spectroscopy (EIS).22,36 Another side effect that needs to be

taken into account in the design of sensing electrodes is the bipolar electrode effect, wherein faradaic reactions occur at the edges of a floating electrode, when the induced potential drop between the solution and the floating electrode is larger than the standard potential of water electrolysis.37−39 To prevent this, an upper limit on the width of the sensing electrodes should be set to ensure that the potential drop along its width will be smaller than the activation voltage for the electrolysis of water.37 In addition to local EIS measurements, another approach taken herein was the global measurement of the current across the membrane, which is inversely proportional B

DOI: 10.1021/acssensors.9b00431 ACS Sens. XXXX, XXX, XXX−XXX

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ACS Sensors

Figure 2. Sketch of the numerical simulation domain and boundary conditions in the microchannel-membrane flow sensor device. extrapolated based on the measured fluorescence intensity of Dylight molecules and normalized by the concentration intensity of the plateau. Local Impedance Measurements. In order to perform continuous local impedance measurements, a single-frequency EIS was performed using two sensing microelectrodes. EIS measurements were collected using a Gamry 3000 potentiostat, covering a frequency ranged from 1 MHz down to about 0.5 Hz. After determining the working frequency (30 kHz) at a specific electrolyte conductivity (σ ∼ 200 μS cm−1), according to the intersection point of the Warburg branch with the real impedance axis in a Nyquist plot, single-frequency EIS was employed to sense a transient local impedance response (modulus) using an AC signal of 50 mV for 500 s (Figure S1). The specific choice of the working frequency is of little significance, as the flow is estimated from the time to steady-state under stepwise CP/flow. In addition, two Gamry devices were simultaneously used to measure two different electrode pairs for differential sensing (i.e., upstream and downstream of the Nafion membrane) or downstream time-of-flight sensing.

to the resistance between the powered electrodes (i.e., anode and cathode). Unlike local electrical measurements, global measurement allows the use of DC measurements, as gas products of water electrolysis can be easily evacuated from the reservoirs. In order to realize both local and global electrical measurements for estimation of the flow intensity, we designed, fabricated, and tested a unique microfluidic device with sensing electrodes embedded within the microchannel and with powered electrodes (anode/cathode) within the inlets of the microfluidic channel and of the side channels.

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EXPERIMENTAL SECTION

Chip Design and Fabrication. The microchannel-membrane device consisted of a straight, long microchannel (main channel), two side microchannels, and an interconnecting Nafion membrane between the channels (Figure 1a).34 In the main channel, which acted as an anode channel, the applied tangential flow deformed the depletion layer and the resulting changes of the local solution conductivity were detected by an embedded electrode array. The side channels, which acted as cathode channels, contained a stagnant electrolyte solution and inlets for the powered external electrodes for inducing the CP. The polydimethysiloxane (PDMS) microchannels (300 μm wide and 40 μm deep) and microelectrode array were fabricated using soft and standard photolithography, respectively, as previously described.34,37 A patterned Nafion membrane (∼300 μm wide and 400 nm high) was placed at the center of the main channel, connecting the main and side channels for the generation of CP. Two 10-paired electrode arrays (20 μm wide, 40 μm gap between pairs) were embedded, with a distance of S ∼250 μm between the Nafion membrane edge and the first electrode pair (Figure 1b). The sensing electrode pairs were positioned symmetrically relative to the center of the membrane, thus enabling simultaneous local EIS measurements both downstream and upstream of the Nafion membrane. Experimental Setup for CP Generation. For generation of CP, four platinum wire electrodes (0.5 mm in diameter) were inserted into the inlets of the main and side channels and connected to a voltage source (Keithley 2636). An external DC voltage was applied to two ends of the main microchannels, while the two side microchannels were electrically grounded (Figure 1b). The examined electrolytes were potassium chloride (KCl) solutions, with measured conductivities of 200 μS cm−1 and 1 mS cm−1, or insulin solution human (Sigma), with a conductivity of 2.1 mS cm−1 at room temperature (20 °C). To visualize the depletion layers, ∼10 μM DyLight 488 negatively charged fluorescent dye (ThermoFisher Scientific), was added to the electrolyte solution so as to closely reflect, excluding their possible third species behavior,34,40 the concentration-polarization of the electrolyte dominant ionic species. All experiments were recorded using an Andor Neo sCMOS camera attached to a Nikon TI inverted epi-fluorescence microscope. The flow in the main channel was achieved by hydrostatic pressure with flow rates lower than ∼1 μL min−1. The resulting flow velocities were extrapolated by analyzing the motion of 1-μm-diameter fluorescent green tracer particles (ThermoFisher Scientific), of 0.002% volumetric concentration, using IMAGE-J software and the particle tracking method. Two experimental modes, i.e., stepwise CP generation (7 V) under steady-state flow and stepwise flow under steady-state CP, were tested. Ionic concentration profiles were

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NUMERICAL SIMULATIONS System Setup. Herein, we used the numerical simulations only for qualitative comparison with the experiments so as to gain physical insights. Hence, we have used several simplifications in the numerical simulations relative to the real device that significantly reduce the computational load, including two-dimensional (2D) geometry, local electroneutrality, and neglect of electro-osmotic flow (EOF) effects. The latter is justified since these are expected to be small due to the symmetrical application of the voltage at the device inlets. In addition, such EOF can be completely suppressed, even for a nonsymmetric application of the voltage, if a proper surface treatment (e.g., polyvinylpyrrolidone (PVP)) is performed to suppress surface native charge.34 The computational model of the microchannel-membrane device, shown in Figure 2, contained two cation exchange membranes (CEMs) of length 2l, that were embedded into the middle of the walls of a straight long microchannel of length 2L and width H. The CEMs were assumed to be ideally permselective with a fixed charge N > 0 and to only permit the passage of cations. At the inlet of the microchannel (x = −L), the concentrations were fixed, and equal to the stirred bulk concentration in the reservoir. The electric potentials at both ends, inlet and outlet, of the microchannel were set to V, but were grounded at the CEMs surfaces. In addition, forced longitudinal flow was imposed within the microchannel. Governing Equations. For a symmetric and binary electrolyte of equal ionic diffusivities (D̃ + = D̃ − = D̃ ), the corresponding setup was modeled using the following dimensionless mass conservation (Nernst−Planck) equations (tilde notations are used below for the dimensional variables, as opposed to their untilded dimensionless counterparts) C

DOI: 10.1021/acssensors.9b00431 ACS Sens. XXXX, XXX, XXX−XXX

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Figure 3. Experimental stepwise CP (7 V) under various steady-state flows. (a) Microscopic images of the CP layer under different steady-state flow rates (from right to left), 60 s after CP initiation (σ ∼ 200 μS cm−1). (b) Representative transient local impedance measurement at the first and fifth sensing electrode pairs. The averaged base impedance before CP generation was 330 KΩ. (c) Steady-state (after ∼500 s) local impedance and propagation velocities of the depleted layer as a function of flow rate measured at various sensing electrode pairs. (d) Transient response of the current across the membrane. The solution used here was insulin solution (σ = 2.1 mS cm−1). The inset microscope images depict corresponding CP behaviors at steady-state, 200 s after CP activation. (e) Steady-state resistance and time to steady-state versus flow rates.

∂c ± = −∇·j± ∂t

t=

(1)

j+ = −(∇c + + c +∇φ) + u(y)c +

(2)

j− = −(∇c − − c −∇φ) + u(y)c −

(3)

(4)

For simplicity, we disregard the possible electro-osmotic flow in the system and impose longitudinal flow of the fully developed Poiseuille velocity profile 2

u = 4um( −y /H + y/H )

c ±̃ c0̃

φ=

̃ ̃ Fφ ̃ ̃ RT

u=

uL ̃ ̃ D̃

are, respectively, the dimensionless time, concentrations of cations and anions, the electric potential, and the longitudinal velocity, with D̃ the diffusion coefficient, c̃0 the stirred bulk concentration at the inlet of the channel used for normalizing the concentrations, F̃ the Faraday constant, R̃ the universal gas constant, and T̃ the absolute temperature. Spatial variables in eqs 2−3 were normalized by one-half of the microchannel length L̃ . Boundary Conditions. The system was supplemented by the following boundary and interface conditions: At the membrane surfaces, we assumed disappearance of anion flux across the membrane

Assuming local electroneutrality, we obtain

2

c± =

(6)

where the dimensionless ionic fluxes j+ and j−are

c+ = c− = c

̃ ̃ tD L̃ 2

cy− − c −φy = 0

(5)

(7)

and continuity of the electrochemical potential of cations (capable of penetrating the interface)

with um being the maximum of the velocity profile. Here, D

DOI: 10.1021/acssensors.9b00431 ACS Sens. XXXX, XXX, XXX−XXX

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ACS Sensors

Figure 4. Numerical simulations of a stepwise CP (V = 15) under various steady-state flows. (a) CP layer under different steady-state flow rates (from left to right) after t = 1 from CP initiation. (b) Transient local resistivity (c−1) responses downstream of the membrane at the sensing electrode (x = 0.143), under various flow intensities. Inset: same plot for early time points. (c) Steady-state local resistivity response and propagation velocities of the depleted layer as a function of flow rate for cin = 1 at the reservoir; L is the distance between the downstream edge of the CEM and sensing electrodes. Inset: measured propagation velocities of the depleted layer as a function of applied flow rate for different solution ionic concentrations. (d) Transient response of the current computed across the membrane. (e) Steady-state resistance and time to steady-state versus flow rates for cin = 1. Inset: same plot for different concentrations.

ln c + + φ = ln N

At the inlet boundary, the concentration of the electrolyte was assumed to be identical to those in the reservoir and the electric potential was V

(8)

across the discontinuities of the electric potential and ionic

c + = cin

concentration, modeling the electric double layer in the electroneutral problem. At the microchannel walls, imperme-

φ=V

(10)

where cin is the normalized (by c̃0) ionic concentration at the inlet reservoir of the microchannel. At the outlet boundary, the electric potential was V

ability to all ions was assumed cy+ = cy− = φy = 0

c − = cin

φ=V

(9) E

cx+ = cx− = 0

(11) DOI: 10.1021/acssensors.9b00431 ACS Sens. XXXX, XXX, XXX−XXX

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ACS Sensors Numerical Method. By adding and subtracting the ion conservation eq 1 for the two ionic species, upon using eqs 2 and 3, and applying the electroneutrality condition eq 4, we arrived at the following equations and boundary conditions: ct = Δc − u(y)cx

∇·(c∇φ) = 0

c = cin

φ=V

x = −L

cx = 0

φ=V

x=L

over the sensing electrodes (see Figure S2). This stands in qualitative agreement with the numerical simulation results of the transient local resistivity (proportional to c−1) response (Figure 4b) at the local impedance sensing location (Figures 3a and 4a). The steady-state values of the local impedance at different sensing electrodes exhibited a strong dependency on the flow rate, wherein their values decreased with increasing flow rate (Figure 3c and Figure 4c). Below flow rates of ∼20 μm s−1 and for sensing electrodes closer to the membrane, the experimentally measured steady-state local impedances exhibit an increasing noise (i.e., error bars), suggesting a lower limit on the range of measured flow rates. In contrast, the numerically computed local impedance (Figure 3c) do not exhibit such noise, suggesting that this might result from the near membrane electroconvective instabilities11,14,18 not accounted for in the numerical simulations and that are expected to influence only the sensors that are in the immediate vicinity of the membrane. As clearly seen, the noise decreases with increasing distance of the sensing electrodes from the membrane interface. Another approach for flow sensing collects time-of-flight measurements using downstream impedance sensors. The propagation velocity as a function of applied flow rate was estimated by ΔL/tr, where ΔL is the distance between the membrane interface and the sensing electrode pair and tr is the impedance signal rising time (i.e., when the impedance exceeds 50% of its initial undisturbed value). As seen in Figure 3c and Figure 4c, sensing electrodes measured a clear correlation between ΔL/tr and the applied flow rate, hence proving the robustness of this flow-sensing approach. Such behavior is expected when the depletion layer propagation is dominated by advection as opposed to diffusive growth (i.e., Ldiff ≈ Dt , see Figure S3). However, at sufficiently high flow rates (i.e., above 60 μm s−1), the depletion layer becomes less sensible (see the associated decrease of the steady-state impedance) and dependent on the location of the sensing electrodes. Generation of the depletion layer resulted in an increase of the overall system resistance, hence, its chronoamperometric (I−t) response exhibited a monotonic decrease in the current from its initial Ohmic response, where Iohm (1.72 ± 0.08 μA) and Rohm (∼ 4.07 MΩ) are the corresponding Ohmic current and resistance (Figure 3d). Since the tangential flow affects the CP layer, the resistance correlated with the applied flow rate intensity. Figure 3d,e and Figure 4d,e depict the I−t responses under stepwise application of CP and steady-state flow, where the overall system resistance at steady-state, Rss, and the time to steady-state, tss, strongly correlated to the flow rate and both decreased with increasing flow rates. The sensing range was limited by sufficiently high flow rates (>90 μm s−1), where the change of Rss and tss values become too small. The corresponding microscopic images are depicted in the inset of Figure 3d. From analyzing both the time to steady-state current response and the steady-state resistance, it seems that the direct I−t measurement may produce a simple and robust flow-sensing method, without the need for an array of sensing electrodes. From the numerical simulations performed for varying solution conductivities, the global (i.e., across the membrane) electrical (I−t) response (inset of Figure 4e) seems to exhibit a weaker dependency on the solution conductivity relative to the local electrical impedance (inset of Figure 4c), which can be accounted for by proper

(12)

0